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TRANSMISSION MECHANISM] Filed Aug. 6, 1938 i '7 Sheets-Sheet 7 T" T F i 110 Patented May 28, 1940 TRANSMISSION MECHANISM Harold Sinclair, Kensington, London, and Robert Cecil Clerk, Egham, England, assignors to Bydraulic Coupling Patents Limited, London, England, a company of Great Britain Application August 6, 1938, Serial No. 223,393 In Great Britain April 25, 1938 16 Claims.

gearing having an improved form of positive gear-selecting clutch.

' Another object is to provide change-speed gearing providing three or more forward speeds through positive gear-selecting means, in which all changes between said speeds can be synchronously effected while the gearing is running and without necessarily disengaging a main clutch associated with the gearing.

Yet another object is to provide change-speed gearing having an improved form of self-shifting positive clutch capable of selectively establishing at least three different speed ratios.

According to the present invention, a synchrocoupling comprises a toothed first member (which is the driving or the driven member), a second member (which is the driven or the driving member). an intermediate member having teeth engageable with the teeth of said first member, said intermediate member being so engaged with said second member as to be constrained to move helically with respect thereto upon relative rotation between said intermediate and second members, and said intermediate member being capable, in the course of such helical movement, of axial displacement relative to said first member from a first position where said teeth are disengaged, through a second position where said teeth are engaged, to a third position where said teeth areagain disengaged, the coupling also comprising two subsidiary ratchet drives adapted to couple said first and intermediate members and serving to effect engagement of the teeth of said fi st and intermediate members without clashing by moving the intermediate member from said first and third positions respectively to said second position, upon relative angular displacement of said first and second members in the two directions respectively, and control means operable for preventing said intermediate member from passing beyond said second position when moved from at least one of said first and third positions.

The improved synchro-coupling may have its intermediate member movable on the second member to a fourth position in which neither of said ratchet drives is operative so as to avoid clicking of the pawls, control means being provided whereby said intermediate member can be moved out of said fourth position to said third position.

The invention will be described, by way of example, with reference to the accompanying drawings relating to a change-speed gearing yielding three forward speed ratios and reverse and suitable for use on a road vehicle driven by an internal-combustion engine.

Fig. 1 is a sectional side elevation of the gearing, with part of the control mechanism omitted and with reverse idler gears in developed view,

Fig. 2 is a sectional end elevation of a detail to a larger scale, taken on the line 2-2 in Fig. 1,

Fig. 3 is a developed section taken on the line 33 in Fig. 1,

Fig. 4 is a sectional end elevation on the line 4-4 in Fig. 1,

Fig. 5 is a sectional side elevation of a detail appearing in Fig. 1,

Figs. 6, 8, 10, 12, 14 and 16 are sectional side elevations of a portion of the gearing, showing the parts in different configurations respectively,

Figs. 7, 9, 11, 13, and 17 are developed sections taken in the same way as Fig. 3, but showing the parts in the configurations of Figs. 6, 8, 10, 12, 14 and 16 respectively.

Fig. 18 is a sectional side elevation of a gearchange lever assembly, taken on the line Ill-l8 in Fig. 19,

Fig. 19 is a sectional end elevation taken on the line Ill-I9 in Fig. 18, and

Fig. 20 is a sectional plan taken on the line 20-20 in Fig. 18.

The change-speed gearing shown in Fig. l. includes a casing 10 in which is journalled an input shaft II, the left hand end of which is supported by a journal bearing not shown. The shaft II is conveniently the driven shaft of a main clutch, -such as hydraulic coupling having a drag-release control as described in Patent No. 1,978,172, or a centrifugal friction clutch, connecting the engine to the gearing. A pinion l2 fixed to the shaft II is in constant mesh with a gear wheel I3 fixed to a live countershaft ll. A middle-speed gear wheel 15 is journalled on the input shaft H by ball bearings l6 and I1 and meshes with a gear wheel l8 which is fixed to the countershaft l4 and which is smaller in diameter than the wheel l3. An output shaft I9 is journalled at 20 in the casing l0 and its lefthand end is supported by a spigot bearing 2| in a bore in the input shaft A sleeve 22 is Journalled by needle roller bearings 23 on a bush 24 on the output shaft. together with the inner race of the bearing and a locking bush 25, between a split ring 28 and a nut 28. The ring 28 is located in a cirreverse gear wheel 33 having internal splines 34 is slidably mounted on splines 35 on the sleeve 22, and its splines 34 are adapted to engage splines 36 on the wheel 3|. A selector rod 31 is provided with a fork 38 engaged in a circumferential groove 39 in the hub of the wheel 33.

A reverse idler cluster consists of two gear wheels 40 and 4| (shown by clotted lines in developed view). These gear wheels are journalled on a fixed shaft the axis of which is denoted by 42. The wheel 4| is in constant mesh with the wheel 32, while the wheel 40 is adapted to be engaged by the slidable reverse wheel 33. when the selector 3! is in its extreme left-hand position. When this selector is in the neutral position, the reverse wheel 33 is disengaged from both the wheels 40 and 3|. 5

The clutch mechanism for selecting the forward gears includes an intermediate member in the form of a nut 50 which is engaged with the splines 21 on the output shaft so that it is constrained to move helically on this shaft between the ring 26 and a split ring 5| retained in a groove 52 by a flanged ring 53 and a snap ring 54. The nut 50 has a flange 55 provided on its circumference with radial teeth 56. When the nut 50 is in its extreme left-hand position, the teeth 56 mesh with teeth 51 formed on a drum 58 which is integral with the input shaft I. To an extension 59 of the wheel I 5 is fixed a ring 50 having internal teeth 6| adapted to be engaged by the teeth 56 as the nut moves to the right from the position shown. On one end of the sleeve 22 is a ring of teeth 62 adapted to be engaged by the nut teeth 56 when the nut is in its extreme right-hand position.

On the left-hand face of the flange 55 are one or more pawls 63 journalled on pins 64. The noses 65 of these pawls (Fig. 2) point in the normal direction of rotation (indicated by the arrow on the input shaft II in Fig. 1) and project slightly in advance (in a circumferential direction) of the leading faces of the teeth 56 that are nearest to them. The tooth clearances and the amount by which the pawls project are exaggerated in Fig. 2. Springs 56 bias the pawls outwards to the position shown. On the right-hand face of the flange 55 are one or more pawls 6'! which are arranged similarly to the pawls 63, except that their noses trail with respect to the said direction of rotation and are slightly behind the trailing faces of the teeth 56 that are nearest to them.

Means for locking the nut 50 in diiferent gear positions will now be described. The helix angle of the splines 21 on the shaft I9 is such that the nut 50 is constrained to rotate through 45 relatively to the shaft l9 when it is displaced through its full travel on this shaft, that is from contact with the stop ring 216 to contact with This bush is clamped,-

the stop ring 5|. The nut 50 has eight axial uniformly distributed splines 59A each having a width equal to the width of the grooves between them. These splines are in constant mesh with eight alternate long and short splines 10A and 10B on the interior of a locking sleeve 10.

The locking bush 25'is provided with two rings of short axial splines 25A and 25B spaced by a circumferential gap 25C. There are eight uniformly distributed splines 253 each having a width (in the circumferential direction) equal to that of the grooves between them, and there are four uniformly distributed splines 25A having the same width as the splines 25B and so positioned as to register, in an axial direction, with grooves between the splines 253 (Fig. 3.)

A second locking sleeve H is slidably mounted on the sleeve 10, and is provided with internal axial splines IIA in constant mesh with splines 19C on the sleeve I0, whereby the sleeves 10 and 1| are prevented from rotating relatively to each other. The sleeve 1| is also provided with eight uniformly distributed axial splines fl IB each having a width equal to that of the grooves between them. The splines "B are so positioned as to register, in an axial direction, withthe grooves between the splines 19A and 10B, and their length is such that they can be accommodated in the gap 25C. When the mechanism is in the configuration shown in Figs. 1 and 3, the end faces of the splines 10B half overlap, in the circumferential sense, the end faces of the splines 25A.

The locking sleeves Ill and II are provided with shoulders I2 and I3 co-operating with the internally flanged ends of a selector tube 14 housed in the sleeve 22. A selector ring 15 is slidably mounted on the sleeve 22, which is provided with three slots 16 (Fig. 4). Each of these slots accommodates a pin 11, by which the tube 14 is rigidly connected to the ring 15. A selector fork 18, mounted on a selector rod I9, is engaged in a groove in the ring 15. The locking sleeves I0 and II are urged into the relationship shown in Fig. l by three light helical compression springs 8|. These springs are guided by pins 82 screwed to the sleeve 10 at the bottom of counterbores 83. Three radial pins 84 are fixed in the sleeve H and project into grooves 85 in the sleeve 10. These grooves are of such a length that the pins 84 prevent the sleeves 10 and H from being forced by the springs 8| far enough apart for both the shoulders 12 and 13 to rub simultaneously against the flanged ends of the tube 14.

When the parts are in the configuration shown in Fig. 1, the axial space between the ends of the splines 10A and 25A is somewhat shorter than the axial space between the ends of the splines 'HB and 25B. Consequently, when the selector tube 14 is urged to the right to cause the splines 10A to abut the ends of the splines 25A, there will still be a substantial axial space between the ends of the splines 'HB and 2513.

The control mechanism (Figs. 1,5, 18, 1,9.and 20) includes a gear-change lever flxedto a shaft 9| slidably journaled in a bracket QZ which is mounted in the vehicle in fixed relationship to the gear casing l9. To the lower end of the lever 90 is fixed a pin 93 engageable alternatively in holes 94 and 95 formed respectively in arms 98 and 99 which are pivotally mounted on the bracket 92 so as to rock in planes parallel to the plane of movement of the lever 90. The arms 98 and 99 are pivotally connected respectively by links I00 and HM to the selector rods 19 and "forward,"

31. T e rod 31 has three notches 31A, 31B and 31C co-operating with a spring-loaded locating plunger 96 and corresponding respectively to the neutral and reverse positions. The selector rod 19 has three notches 19A. 19B and 19C co-operating with a spring-loaded locating plunger 91 and corresponding respectively to high, low and middle speed positions.

A resilient connection is provided between the selector rod 19 and the fork 18. As shown in Fig. 5, a helical spring I02 is placed round the rod 19, being normally partly compressed between two collars I03 and I04 slidable on the rod 19 and within a tubular body I05 of thefork I6. Snap rings I06 and I01 are engaged in circumferential grooves .in the rod 19 and form abutments for the collars I03 and I04 respectively. One end of the tube I05 has an inturned flange I08 shaped to engage the collar I04 and to clear the snap ring I01 and the other end of the tube is provided with a hollow screwed plug I09 shaped to engage the collar I03 and to clear the snap ring I06. If the rod 19 is moved to the right and the fork is prevented from following, the snap ring I06 forces the collar I03 to the right and further compresses the spring I02 against the collar I04 which bears against the flange I08. When the fork is free to move, the spring I02 expands, forcing the fork to the right until the collar I04 is positively arrested by the snap ring I01. A similar action can take place. when the rod is moved to the left.

The gear lever 90 operates in a gate 0 hav ing the shape shown in Fig. 20 and providing a continuous passage from reverse (R), through neutral (N), low (I) and middle (2) speed positions, to the high-speed position (3).

The strengths of the various springs are so selected that the compression of the spring I02 of the selector fork 18 when in its normal position, as in Fig. 5, is sufficient to maintain compressed the springs 8| of the locking sleeves; and the restraint imposed to movement of the selector rod 19 by the spring-loaded locating plunger 91 is sufiicient to hold this rod stationary when the spring I02 is fully compressed.

The gearing operates as follows.

In starting from rest on low speed, the gear lever is in the position shown in Figs. 18, 19 and 20. When the main clutch (not shown) engages, the input shaft II rotates and causes the teeth 51 to rotate with the shaft II, the teeth 6| to rotate forwards at a lower speed, and the sleeve 22 with the teeth 62 to rotate forwards at a still lower speed. If the nut happens to be in its extreme left-hand position, the teeth 51 cooperate with the teeth 56 to move the nut to the position shown in Fig. 1. Then one of the pawls 61 is engaged by a tooth 6| which moves the nut to the right until the teeth 56 and 6| engage. The movement continues until one of the pawls 61 engages a tooth 62 which draws the nut against the stop 26 and brings the teeth 56 and 62 into mesh. Power is now transmitted through the gear wheels I2 and I3, the countershaft I4, the gear wheels 32 and 3|, the gear wheel 33 acting as a jaw clutch, the sleeve 22 and thence through the nut 50, the splines 21 and the stop 26 to the output shaft I9.

To change to middle speed, the gear lever is moved to position 2 (Fig. 20) causing the selector rod 19 to move to the right until the notch is engaged by the locating plunger 91. Since the'ends of the splines 10A on the locking sleeve 10 abut against the ends of the splines 25A on the'locking bush, as shown in Figs. 6 and 7, the spring I02 of the selector fork 18 is compressed. The engine is now retarded. As the inertia of the vehicle causes the output shaft |9*to continue rotating without substantial fall of speed, the teeth 62 on the low-speed sleeve 22 co-operate with the nut teeth 56 to move the latter to the left until the pawls 63 come in the path of the middle-speed teeth 6|. the teeth 6| ratchet over the pawls 63. but, as the engine speed continues to fall and in consequence the speed of the teeth 6| tends to drop below that of the nut 50, a tooth 6| engages a pawl 63 and causes the nut to move farther to the left until the teeth 56 mesh with the teeth 6|.- These teeth co-operate to maintain the travel of the nut tov the left.

This movement of thenut to the left has caused the locking sleeves 10' and 'II to rotate backwards relatively to the locking bush 25 on the output shaft, and, at the instant when the teeth 56 and 6| come fully into mesh, as will be clear from Fig. '1, the longsplines 10A on the locking sleeve 10 slip round the trailing edges of the splines 25A and permit the locking sleeves 10 and 1| to move to the right under the influence of the selector spring I02. At this instant the splines 1|B on the locking sleeve 1| are in register with the grooves between the splines 253 on the locking bush 25. However, since the action of the teeth 56 and 6| maintains the movement of the nut to the left and hence its backward rotation relative to the locking bush 25, and since, at the instant when the splines 10A slip round the splines 25A, there is still a substantial axial space between the splines HE and 253, during the time required for the selector mechanism to operate to move the locking sleeve 1| far enough to the right to take up this axial space, the relative rotation of the nut and. the output shaft has brought the splines 1|B into such a position that their ends abut against the ends of the splines 25B, so that the locking sleeve 1| is prevented from moving farther to the right, the two locking sleeves 10 and 1| being telescoped together, compressing the springs 8|. When the nut reaches the position shown in Fig. 8, its movement to the left ceases, and the pawls 61 ratchet over the teeth 6| since, owing to the continued retardation of the engine, the teeth 6| are now rotating slower than the nut. At the same time, as the teeth 51, which rotate with the input shaft, are still rotating faster than the nut 50. the pawls 63 ratchet over the teeth 51. The four long splines 10A on the locking sleeve 10 are now spaced circumfercntially from the teeth 25A on the locking bush 25, as shown in Fig. 9.

The engine is now accelerated again before the speed of the shaft II has fallen to that of the shaft I9, and as soon as the middle-speed teeth 6| tend to rotate faster than the output shaft I9, one of the pawls61 is engaged by a tooth 6| and the nut 50 is caused to move to the right, the nut teeth 56 meshing with the teeth 6 The consequent forward rotation of the nut 50 on the output shaft I9 causes the leading faces of the long splines 10A to approach the trailing faces of the splines 25A, and, as the teeth 56 and 6| become fully meshed (Fig. 10), the splines 10A engage the splines 25A Fig. 11) and these splines thus cooperate to prevent further movement of the nut to the right. At the same time the splines 1IB come into register with the grooves between the splines 25B, and the springs Bl cause the locking sleeve 1| to move to the right until the splines 25B and NB are in mesh with each other. The gearing is now in condition for transmitting both driving and .reverse torque (namely, the torque in the sense that occurs when the engine is retarded and power is temporarily transmitted from the output shaft to the engine) on the intermediate-speed ratio, the driving power being transmitted from the shaft II by the wheels I2, l3, l8 and I5, the nut 50 and the locking splines 50A, 10A and 25A to the output shaft I I It will be evident that the operation of changing from low/t6 middle speed involves maintaining the middle-speed coupling 56, 61 in a bi directionally-free condition with the locking sleeves in the positions shown in Figs. 6 and '7 until the ratio of speeds of the input and output shafts has fallen to a value between the highand middle-speed ratios, then automatically converting the middle-speed coupling into its freewheeling condition, as shown in Figs. 8 and 9, accelerating the engine until this free-wheel coupling engages, and finally looking it bidirectionally, as shown in Figures 10 and 11. As the drive is picked up by engagement of a freewheeling coupling, and as this coupling cannot be bidirectionally locked unless its driving and driven parts are running in synchronism, there is no risk of shock engagement.

To change from middle to high speed, the gear lever 90 is moved, while the engine is still transmitting power, to position 3 (Fig. 20), causing the selector rod 19 to move to the position where the notch 19A is engaged by the locating plunger 91. As the splines HE and 25B are not transmitting torque, the locking sleeve 11 is free to move to the left until the splines 11B abut the splines 25A, the difference between the travel of the selector rod 19 and the locking sleeve 11 being accommodated by the: selector spring I02. As the locking sleeve 10 is still transmitting driving torque, it remains held by friction in the position shown in Figs. 10 and 11, the springs 8| being compressed. The engine is now retarded. As soon as the driving torque vanishes, the looking sleeve 10 is moved by the springs 81 to the position shown in Fig. 12. As the speed of the input shaft ll falls relatively to that of the output shaft 19, the middle-speed teeth 6| co-operate with the nut teeth 56 to move the nut to the left to the position shown in Figs. 1 and 3, the pawls 63 and 61 ratcheting over the teeth 51 and GI. When the speed of the input shaft Ii tends to drop below that of the output shaft, one of the pawls 63 engages a tooth 51 and causes the nut to move to the left against the stop 51, bringing the nut teeth 55 fully into mesh with the high-speed teeth 51. The input and output shafts are now maintained in synchronism by reverse torque. the nut 50 co-operating with the high-speed teeth 51 to form an inverted unidirectional coupling. The condition of the parts just as synchronism is being attained is shown in Figs. 12 and 13. Immediately synchronism has occurred, the splines 1lB register with the grooves between the splines 25A, 50 that the selector spring 102 can operate to bring the locking sleeves 10 and 1| into the positions shown in Figs; 14 and 15, with the splines 25A and 11B meshed together. The nut 50 is thereby locked in the extreme left-hand position, and when the engine again generates power the nut is prevented from moving on the output shaft and the drive is transmitted from the shaft II to the nut 50 and thence to the output shaft 19 through the locking sleeves 10 and 11 and the splines 1 IB and 25A.

The described means of engaging the highspeed drive form the subject of patent application Serial No. 169,973, filed October 20, 1937.

To change from high to middle speed the gear lever 90 is moved to the middle-speed position and the engine is momentarily retarded to unload the locking splines HE and 25A and permit the locking sleeve 1| to move to the right under the influence of the springs 8|. The engine is then accelerated, and the high-speed teeth 51 co-operate with the nut teeth 56 to move the nut to the right, the middle-speed engaging as hereinbefore described with reference to 4 Figs. 10 and 11.

To change from middle to low speed, the gear lever is moved to shift the selector rod 19 to the position shown in Fig. 1 and thus bring the splines 1IB into the circumferential gap 250. The locking sleeve 10, owing to the drive torque transmitted by it, is momentarily held by the friction acting between the splines in the position shown in Figs. 10 and 11, and the springs 8| are compressed. The engine is now momentarily retarded to remove the drive torque and allow the locking sleeve I to be moved by the springs 81 to the position shown in Fig. 1, and as the engine is again accelerated, the low speed is established as hereinbefore described.

To obtain reverse, the output shaft being stationary and the gear lever in position I, the input shaft l l is allowed to rotate forwards just far enough to establish low speed and the gear lever 90 is thereupon moved to the position R. As the gear lever moves into contact with the portion 112 of the gate (Fig. 20), the selector rod 19 is moved thereby to the position where its notch 19A is en gaged with the locating plunger 91. The locking splines 25A and 11B are thereby meshed together, as shown in Figs. 16 and 17, the nut teeth 56 remaining in mesh with the low-speed teeth 62. As the gear lever moves sideways into contact with the portion HI of the gate, the pin 93 on the gear lever (Fig. 19) disengages from the hole 94 in the arm 98 and engages in the hole 95 in the arm 99. As the gear lever finally moves to position R, it shifts the selector rod 31 through the neutral position to the position where the notch 31C is engaged with the locating plunger 96. The slidable gear wheel 33 is thereby slid out of engagement with the wheel 31 and into mesh with the reverse idler 40. When the main clutch is re-engaged, the drive is transmitted from the shaft ll through the wheels 12 and 13, the countershaft I 4, the wheels 32, 41, 40 and 33, the sleeve 22 and the nut 50, and thence through the locking sleeves and 1| and the splines TIB and 25A to the output shaft l9.

In some constructions embodying the improved synchro-coupling, it is desirable to omit the sleeve 22 and the teeth 62 from the design shown in Fig. 1. Consequently when the locking sleeves are in the configuration shown in Fig. 1, and the teeth 6| are rotated forwards faster than the output shaft 19 so that the nut is rotated forwards relatively to the output shaft, the inertia of the nut will carry it to the right against the stop 26 after the teeth 56 have disengaged from the teeth 6|. Under these conditions not only is the coupling bidirectionally free, but all the pawls are out of the paths of teeth and do not click. The coupling can be restored to an operative condition by operating the control means to move the selector tube I4 to its extreme lefthand position. This causes the left-hand end of the locking sleeve 18 to engage the pivot pins of the pawls 61 and thereby move the nut to the If it is desired to obtain at will a free-wheeling action between the driving members 59, 60 and the output shaft i9 (which may be convenient where the driving member 58 and teeth 51 are omitted and the extreme left-hand position of .the nut is that shown in Fig. 1) the lengths and axial positions of the locking splines may be soselected as to permit the right-hand ends of the splines 10A to mesh with the left-hand ends 01' the splines 25A while the splines "B are still in the circumferential gap 25C.

We claim: I

1. A synchro-coupling comprising a toothed first member, a second member, an intermediate member having teeth engageable with the teeth of said first member, said intermediate member being so engaged with said second member as to be constrained to move helically with respect thereto upon relative rotation between said intermediate and second members, and said intermediate member being capable, in the course of such helical movement, of axial displacement relative to said first member from a first position where said teeth are disengaged, through a second position where said teeth are engaged, to a third position where said teeth are again disengaged, the coupling also comprising two subsidiary ratchet drives capable of coupling said first and intermediate members and serving to effect engagement of said toothed members without clashing by moving said intermediate member from said first and third positions respectively to said second position, upon relative angular displacement of said first and second members in two directions respectively, and control means operable for preventing said intermediate member from passing beyond said second position when moved from at least one of said first and third positions.

2. A synchro-coupling comprising a toothed first member, a second member co-axial with said first member, an intermediate member having teeth engageable with the teeth of said first member, said intermediate member being so engaged with said second member as to be con- -second pawl mounted on one of said toothed members and engageable with teeth on the other of said toothed members, said pawls being so positioned as to efiect engagement of said toothed members without clashing by moving said intermediate member from said first and third positions respectively to said second position, upon relative angular displacement of said first and second members in two directions re-- spectively, and control means operable for preventing said intermediate member from passing beyond said second position when moved from at least one of said first and third positions.

3. A synchro-coupllng comprising a toothed first member, a second member co-axial with said first member, an intermediate memberhavin teeth engageable with the teeth of said first member, said intermediate member being so mounted on said second member as to be constrained to move helically thereon upon relative rotation between said intermediate and second members, and said intermediate member being capable, in the course of such helical movement, of axial displacement relative to said first member from a first position where said teeth are disengaged through a second position where said teeth are engaged, to a third position where said teeth are again disengaged, the coupling also comprising a first pawl mounted on one of said ,toothed members and engageable with teeth on the other of said toothed members, a second pawl mounted on one of said toothed members and engageable with teeth on the other of said toothed members, said pawls being so positioned as to effect engagement of said toothed members without clashing by moving said intermediate member from said first and third positions respectively to said second position, upon relative angular displacement of said first and second members in two directions respectively about their com- 'mon axis,.and a slidable control sleeve operable for coupling together said second and intermediate members with rotational backlash which is taken up' as said toothed members attain full engagement.

4. A synchro-coupling comprising a toothed first member, a second member co-axial with said first member, an intermediate member having teeth engageable with the teeth ofsaid first 7 member, said intermediate member being so mounted on said second member as to be constrained to move helically thereon upon relative rotation between said intermediate and second members, and said intermediate member being capable, in the course of suchhelical movement, of axial displacement relative to said first member from a first position where said teeth are disengaged, through a second position where said teeth are engaged, to a third position where said teeth are again disengaged, the coupling also comprising a first pawl mounted on one of said toothed members and engageable with teeth on the other of said toothed members, a second pawl mounted on one of said toothed members and engageable with teeth on the other of said toothed members, said pawls being so positioned as to effect engagement of said toothed members without clashing by moving 'said intermediate member from said first and third positions respectively to said second position, upon relative angular displacement of said first and second members in two directions respectively about their common axis, a slidable control sleeve operable for coupling together said second and intermediate members with rotational backlash which is taken up as said toothed members attain full engagement, a locking member operable in consequence of such engagement for eliminating said backlash and common control means for actuating said control sleeve and locking member.

5. A synchro-coupling comprising a toothed first member, a second member co-axial with said first member, an intermediate member having teeth engageable with the teeth of said first member, said intermediate member being so mounted on said second member as to be concapable, in the course of such helical movement,

of axial displacement relative to said first member from a first position where said teeth are engaged, through a second position 'where said teeth are engaged, to a third position where said teeth are again disengaged, the coupling also comprising a first pawl mounted on one of said toothed members and engageable with teeth on the other of said toothed members, a second pawl mounted on one of said toothed members and engageable with teeth on the other of said toothed members, said pawls being so positioned as to effect engagement of said toothed members without clashing by moving said intermediate member from said first and third positions respectively to said second position, upon relative angular displacement of said first and second members in two directions respectively about their common axis, and said intermediate member being also movable on said second member to a fourth position in which neither of said pawls is in the path of said teeth with which they cooperate, and control means operable for preventing said intermediate member from passing beyond said second position when moved from at least one of said first and third positions and for moving saidintermediate member from said fourth to said third position.

6. A changespeed gearing yielding at least two speed ratios between its input shaft and its output shaft and comprising a, gear selecting clutch including two members which in a first condition of the clutch are free to rotate in both senses relatively to each other and which are engageable together to establish the higher speed ratio, controllable means for causing said clutch members, in a second condition of said clutch, to engage together automatically to cause transmission of driving torque from said input shaft to said output shaft and to free wheel under torque in the sense opposed to said driving torque, and means for locking said clutch members bidirectionally together so that they can transmit torques in both senses, said controllable means being operable for changing said clutch from said first to said second condition only whilst the ratio of the speeds of said input and output shafts is lower than said higher-speed ratio, and said controllable means including a preselector locking member serving to lock said clutch members bidirectionally. together in consequence of their engagement under driving torque.

'71 A change-speed gearing yielding at least three speed ratios between its input and output shafts, wherein the elements for transmitting power on the high-speed and a middle-speed ratio include a common gear-selecting clutch having three co-axial relatively rotatable members, the first and third of which have teeth, the

first and second of which are engaged together position where said third and intermediate members are in mesh, pawls on at least one of said toothed members positioned to engage teeth on the remainder of said toothed members to effect engagement of said first and intermediate members by moving said intermediate member from said first and third positions respectively to said second position upon the ratio of speeds of said input and output shafts being lower than and exceeding respectively said middle-speed ratio and to effect engagement of said third and intermediate members by moving said intermediate member from said third to said fourth position upon the ratio of speeds of said input and output shafts tending to become lower than said highspeed ratio, and control means operable alternatively for locking said intermediate member in said fourth position and for preventing said intermediate member from passing beyond said second position when moved from said third position.

8. A change-speed gearing as claimed in claim I, wherein said control means include a locking member operable for locking said intermediate member in said second position.

9. A change-speed gearing as claimed in claim backlash.

11. A change-speed gearing as claimed in claim 7, wherein said control means include a first locking member operable for coupling said second and intermediate members together with angular backlash and a second locking member operable for thereafter coupling said second and intermediate members together without substantial backlash.

12. A change-speed gearing as claimed in claim '7, wherein said control means include two locking members constrained to rotate respectively with said second and intermediate members, a selector capable of sliding one of said locking members with respect to the said member with which it is constrained to rotate, and a projection on one of said locking members engageable alternatively in two circumferentially staggered and axially spaced recesses in the other of said locking members.

13. A change-speed gearing as claimed in claim 7, wherein said control means include a locking sleeve slidable on but constrained to rotate with said intermediate member, two axially spaced rings of lockingsplines rigid with said second member, the splines in one of said rings being circumferentially staggered with respect to the splines in the other of said rings, internal splines on said locking sleeve engageable with angular backlash with one of said rings, a second locking sleeve constrained to rotate with and slidable with respect to said first locking sleeve, said second sleeve having internal splines engageable alternatively with said rings, and a selector member operatively connected with said sleeves.

14. A change-speed gearing as claimed in claim 'I, wherein said control means include a first locking sleeve slidable on and constrained to rotate with said intermediate member, a second locking sleeve telescopically mounted on said first sleeve,

resilient means urging said sleeves apart, a se- 75 lector embracing both of said sleeves, two axially spaced rings of locking splines rigid with said second member, the splines in one of said rings being circumferentially staggered with respect to the splines in the other of said rings, internal splines on an end of said first sleeve engageable with angular backlash with one of said rings, and internal splines on said second sleeve engageable alternatively with said rings.

15. Change-speed gearing comprising a first main shaft, a second main shaft co-axial therewith, a countershaft, a gear train connecting said first shaft to said countershaft, a middlespeed gear wheel rotatable about said first shaft and meshing with a gear wheel fixed to said countershaft, a sleeve rotatable about said second shaft, a gear train for connecting said sleeve to said countershaft, a nut engaged with helical splines on said second shaft, said nut having teeth engageable, as said nut moves helically on said splines, with teeth on said sleeve, with teeth on said middle-speed wheel and with teeth on said first shaft, at least one of said toothed members having pawls co-operating with other of said toothed members and positioned to maintain helical movement of said nut in one direction as the relative speed of said main shafts varies in one sense and in the other direction as said relative speed varies in the other sense, and control means operable for locking said nut in at least one limit of its range of movement and for preventing said nut, when moving in one direction at least, from passing beyond the position where its teeth are in mesh with the teeth of said middle-speed wheel.

16. A change-speed gearing having a driving shaft, a driven shaft co-axial therewith, a countershaft, a gear train connecting said driving shaft to said cauntershaft, an intermediate-speed member rotatable about said driving shaft, a gear train connecting said member to said coun-' tershaft, a low-speed train for connecting said countershaft to said driven shaft, a nut engaged on helical splines on said driven shaft and having teeth engageable alternatively with teeth on said intermediate-speed member and with teeth on said input shaft, said teeth being so positioned that, as said nut rotates in the forward direction relatively to said driven shaft said nut is disengaged from said input-shaft teeth, thereafter engaged with said intermediate-member teeth, and finally disengaged from said intermediatemember teeth, pawls on at least one of said toothed members co-operating with the remainder of said toothed members to rotate said nut backwards relatively to said driven shaft and into mesh with the teeth on said driving shaft as the speed of said driving Lhaft falls relative to that of the driven shaft and to rotate said nut forwards and into mesh with said intermediatemember teeth as said relativespeed rises, and control means operable alternatively for locking said nut in engagement with said driving-shaft teeth and for preventing said nut, as it moves away from said driving-shaft teeth, from running through the position where its teeth are in mesh with said intermediate-member teeth.

HAROLD SlNCLAIR. ROBERT cnom CLERK. 

